The Reformation had the impact of splitting the church in Western Europe into many different groups or "denominations," as we now call them. Prior to the Reformation, throughout most of Europe everyone was part of the Roman Catholic Church. (In the East, there had been the previous split-off of the Eastern Orthodox Church, but let's stick to the Reformation era and western Europe in this answer.) Due to the efforts of reformers like Luther and Calvin and others, there now were multiple religious groups in Europe.
This also led to warfare in the 16th and 17th centuries -- the "Wars of Religion," as they've been called. It took some time for people to grow used to the idea that there could be more than one religion in Europe, and the newly Protestant territories had to fight to maintain sovereignty over affairs in their own territories.
